Fast Magic-Angle Spinning Three-Dimensional NMR Experiment for Simultaneously Probing H-H and N-H Proximities in Solids

摘要

A fast magic-angle spinning (MAS, 70 kHz) solid-state NMR experiment is presented that combines H-1 Double-Quantum (DQ) and N-14-H-1 HMQC (Heteronuclear Multiple-Quantum Coherence) pulse-sequence elements, so as to simultaneously probe H-H and N-H proximities in molecular solids. The proposed experiment can be employed in both two-dimensional (2D) and three-dimensional (3D) versions: first, a 2D N-14 HMQC-filtered H-1-DQ experiment provides specific DQ-SQ correlation peaks for proton pairs that are in close proximities to the nitrogen sites, thereby achieving spectral filtration. Second, a proton-detected three-dimensional (3D) H-1-DQ)-N-14(SQ)-H-1(SQ) experiment correlates H-1(DQ)-H-1(SQ) chemical shifts with N-14 shifts such that longer range N center dot center dot center dot H-H correlations are observed between protons and nitrogen atoms with internuclear NH distances exceeding 3 A. Both 2D and 3D versions of the proposed experiment are demonstrated for an amino acid hydrochloride salt, L-histidine center dot HCl center dot H2O, and a DNA nucleoside, guanosine center dot 2H(2)O. In the latter case, the achieved spectral filtration ensures that DQcross peaks are only observed for guanine NH and CH8 H-1 resonances and not ribose and water H-1 resonances, thus providing insight into the changes in the solid-state structure of this hydrate that occur over time; significant changes are observed in the NH and NH2 H-1 chemical shifts as compared to the freshly recrystallized sample previously studied by Reddy et al., Cryst. Growth Des. 2015, 15, 5945.